Continuous production of thermochromic yarns

ABSTRACT

A method of continuous production of thermochromic nanofiber yarns is described. The method includes applying a first thermochromic material to a nanofiber sheet, and then twisting that nanofiber sheet into a first thermochromic yarn. A second thermochromic material is optionally applied to the first thermochromic yarn to produce a second thermochromic yarn. Alternatively, a homogeneous mixture of the first and the second thermochromic materials is applied to a nanofiber sheet in a single layer to produce a thermochromic nanofiber yarn. The thermochromic yarn can be a single ply yarn or a multi-ply yarn depending on the color to be displayed.

TECHNICAL FIELD

The present disclosure relates generally to nanofiber yarns. Specifically, the present disclosure is directed to continuous production of thermochromic yarns.

BACKGROUND

Nanofibers can attain various configurations when fabricated under different conditions. When nanofibers having approximately equal dimensions are arranged substantially parallel to one another on a substrate and in an orientation approximately perpendicular to the surface of the substrate, the nanofibers are referred to as a nanofiber forest or simply a “forest.” Nanofibers can be drawn from a nanofiber forest to form a nanofiber sheet (or simply “sheet”). In a sheet configuration, the nanofibers are aligned end to end in a plane of the sheet. Nanofiber sheets can be spun to further configure them into nanofiber yarns. Nanofiber yarns can be single ply nanofiber yarns that are formed from the true-twisting or false-twisting of a single nanofiber sheet into a single strand of nanofiber yarn. Alternatively, multiple single strand nanofiber yarns can be plied together into a multi-ply nanofiber yarn. Regardless, the various configurations of nanofibers retain fundamental characteristics including electrical conductivity, thermal conductivity, ultimate tensile strength, among others.

SUMMARY

Example 1 is a method comprising: drawing a nanofiber sheet from a nanofiber forest; applying a first thermochromic material to the nanofiber sheet; twisting the nanofiber sheet with the first thermochromic material into a first thermochromic nanofiber yarn; and applying a second thermochromic material to the first thermochromic nanofiber yarn to produce a second thermochromic nanofiber yarn.

Example 2 includes the subject matter of Example 1, further comprising heating the first thermochromic nanofiber yarn before applying the second thermochromic material.

Example 3 includes the subject matter of either of Example 1 or Example 2, wherein heating the first thermochromic yarn comprises heating to a first temperature of from 90° C. to 115° C.

Example 4 includes the subject matter of any of Examples 1-3, further comprising heating the second thermochromic nanofiber yarn after applying the first thermochromic material and the second thermochromic material.

Example 5 includes the subject matter of any Example 4, wherein heating the second thermochromic yarn comprises heating to a second temperature of from 90° C. to 115° C.

Example 6 includes the subject matter of any of Examples 1-5, wherein the first thermochromic material is applied to the nanofiber sheet as a solution of a first solvent and the first thermochromic material.

Example 7 includes the subject matter of any of Examples 1-6, wherein the second thermochromic material is applied to the first thermochromic nanofiber yarn as a solution of a second solvent and the second thermochromic material.

Example 8 includes the subject matter of any of Examples 1-7, wherein the first thermochromic material has a first transition temperature.

Example 9 includes the subject matter of any of the preceding Examples, wherein the second thermochromic material has a second transition temperature different from the first transition temperature.

Example 10 includes the subject matter of Example 9, wherein the second thermochromic nanofiber yarn displays: a first color below the first transition temperature; a second color different from the first color between the first transition temperature and the second transition temperature; and a third color different from the first and second colors above the second transition temperature.

Example 11 includes the subject matter of any of Examples 1-10, further comprising applying power of at least 0.05 Watts/cm to the second thermochromic nanofiber yarn, the applied power causing a transition from a first color to at least one of a second color or a third color.

Example 12 includes the subject matter of any of Examples 1-11, wherein a diameter of the first thermochromic nanofiber yarn and the second thermochromic nanofiber yarn is at least 5 microns.

Example 13 includes the subject matter of any of Examples 1-12, further comprising: drawing an additional nanofiber sheet from the nanofiber forest; applying a third thermochromic material to the additional nanofiber sheet; twisting the additional nanofiber sheet with the third thermochromic material into a third thermochromic nanofiber yarn; and plying the third thermochromic nanofiber yarn with the second thermochromic nanofiber yarn to produce a multi-ply thermochromic nanofiber yarn.

Example 14 includes the subject matter of any of Examples 9-13, wherein the third thermochromic material has a third transition temperature different from a first transition temperature and a second transition temperature.

Example 15 includes the subject matter of any of Examples 1-14, further comprising adding a conductive material to the nanofiber sheet.

Example 16 includes the subject matter of any of Examples 1-15, further comprising adding a metallic nanoparticle to the nanofiber sheet.

Example 17 is a method comprising drawing a nanofiber sheet from a nanofiber forest; applying a mixture of a first thermochromic material and a second thermochromic material to the nanofiber sheet; and twisting the nanofiber sheet with the first and the second thermochromic materials into a thermochromic nanofiber yarn.

Example 18 includes the subject matter of Example 17, wherein the mixture of the first thermochromic material and the second thermochromic material is homogeneous.

Example 19 includes the subject matter of Example 18, the mixture comprises equal parts of the first and the second thermochromic materials.

Example 20 includes the subject matter of Example 19, wherein the mixture of the first thermochromic material and the second thermochromic material is applied to the nanofiber sheet in a single layer.

Example 21 is a textile that includes the subject matter of any of the preceding Examples.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates an example forest of nanofibers on a substrate, in an example.

FIG. 2 illustrates an example reactor used for growing nanofibers, in an example.

FIG. 3 is an illustration of a nanofiber sheet that identifies relative dimensions of the sheet and schematically illustrates nanofibers within the sheet aligned end-to-end in a plane parallel to a surface of the sheet, in an example.

FIG. 4 is an image of a nanofiber sheet being laterally drawn from a nanofiber forest, the nanofibers aligning from end-to-end as schematically shown in FIG. 3.

FIG. 5A is an image of a single ply, false twisted nanofiber yarn, in an example.

FIG. 5B is an image of a multi-ply, false twisted nanofiber yarn, in an example.

FIG. 6 is a schematic diagram showing an example system for producing thermochromic nanofiber yarns, in an example.

FIG. 7 is a method flow diagram for an example method of production of thermochromic nanofiber yarns, in an example.

FIG. 8 is a schematic illustration of a thermochromic nanofiber yarn capable of displaying three different colors within three different corresponding temperature ranges, in an example.

FIG. 9A is a cross-sectional view of the thermochromic nanofiber yarn of FIG. 8 in which two separate layers of thermochromic materials have been applied over nanofiber yarns, in an example.

FIG. 9B is a cross-sectional view of the thermochromic nanofiber yarns of FIG. 8 in which a single layer comprising two different thermochromic materials have been applied over nanofiber yarns, in another example.

The figures depict various embodiments of the present disclosure for purposes of illustration only. Numerous variations, configurations, and other embodiments will be apparent from the following detailed discussion. Furthermore, as will be appreciated, the figures are not necessarily drawn to scale or intended to limit the described embodiments to the specific configurations shown. For instance, while some figures generally indicate straight lines, right angles, and smooth surfaces, an actual implementation of the disclosed techniques may have less than perfect straight lines and right angles, and some features may have surface topography or otherwise be non-smooth, given real-world limitations of fabrication processes. In short, the figures are provided merely to show example structures.

DETAILED DESCRIPTION Overview

Temperature sensitive materials that change color and/or opacity (or equivalently, transparency) upon a change in temperature have many practical applications. In some cases, these “thermochromic materials” include polymers (e.g., liquid crystal polymers among others) and/or pigments (e.g., leuco dyes). Various thermochromic materials can be composed to transition from a first color to a second color when heated or cooled beyond corresponding first and second thermochromic transition temperatures. For example, some leuco dyes can turn from white (or transparent) to blue upon being cooled to a thermochromic transition temperature of less than approximately 5° C. (+/−10° C.) or lower. In another example, a layer that includes suitably composed liquid crystals can transition from black to transparent as the temperature increases above a corresponding thermochromic transition temperature (e.g., above 40° C., above 60° C., or above 100° C.).

Thermochromic materials can be applied directly to a substrate, encapsulated in a carrier (e.g., a leuco dye within a polymer bead or micro-bead), or otherwise combined with a matrix prior to or during application to a substrate. As the temperature of the environment surrounding the thermochromic material changes, the thermochromic material responds by changing color or transitioning to a state having a different degree of transparency (i.e., from opaque to transparent or vice versa). Regardless of the configuration of the thermochromic material or the manner in which it is applied to a substrate, applications of thermochromic polymers have generally included a single reversible color transition from a first color to a second color. Most commercial applications of thermochromic materials involve surfaces having a macroscopic characteristic dimension (e.g., a length of a side, a diameter, or chord) greater than one millimeter (mm). Examples of these include beverage cans (beverages that are heated or cooled above or below ambient temperature), stickers and labels that can be used as thermometers (e.g., a warning label indicating a hot surface), among others.

In accordance with some embodiments of the present disclosure, techniques are described for the fabrication of thermochromic carbon nanofiber yarns having at least two thermochromic transitions from a first color (e.g., transparent) to a second color different from the first color (e.g., blue), and from the second color to a third color (e.g., purple) different from both the first color and the second color. It will be appreciated that these multiple thermochromic transitions can occur upon increasing a temperature of the thermochromic material from a first temperature to a second temperature, and from the second temperature to a third temperature (all different from one another and corresponding to associated thermochromic transition or “threshold” temperatures). In some cases, these transitions can be reversed by decreasing the temperature from the third temperature to the second temperature and then from the second temperature to the first temperature. In some embodiments described herein, thermochromic materials are applied to and/or infiltrated within nanofiber sheets and/or nanofiber yarns. In this way, thermochromic materials can be, for example, woven in fabrics, integrated within displays, or used in other applications in which nanofiber sheets and/or nanofiber yarns are advantageous. Furthermore, because nanofiber sheets and nanofiber yarns (particularly those fabricated from carbon nanofibers) can be thermally conductive, times required to cause thermochromic transitions within the nanofiber sheets and/or nanofiber yarns can be on the order of milliseconds, tenths of seconds, or seconds. This is in contrast to response times for more customary configurations of thermochromic materials (e.g., labels formed from a thermochromic material embedded in a thermally insulating polymer) that do not include carbon nanofibers, which can be on the order of tens of seconds or minutes.

Prior to describing thermochromic displays of the present disclosure, the fabrication and configuration of nanofiber forests, nanofiber sheets, and nanofiber yarns as described.

Nanofibers and Nanofiber Forests

As used herein, the term “nanofiber” means a fiber having a diameter less than 500 nm. While the embodiments herein are primarily described as fabricated from carbon nanotubes, it will be appreciated that other carbon allotropes, whether graphene, micron or nano-scale graphite fibers and/or plates, and even other compositions of nano-scale fibers such as boron nitride may be densified using the techniques described below. As used herein, the terms “nanofiber” and “carbon nanotube” encompass both single walled carbon nanotubes and/or multi-walled carbon nanotubes in which carbon atoms are linked together to form a cylindrical structure. In some embodiments, carbon nanotubes as referenced herein have between 4 and 10 walls.

The dimensions of carbon nanotubes can vary greatly depending on production methods used. For example, the diameter of a carbon nanotube may be from 0.4 nm to 100 nm and its length may range from 10 μm to greater than 55.5 cm. Carbon nanotubes are also capable of having very high aspect ratios (ratio of length to diameter) with some as high as 132,000,000:1 or more. Given the wide range of dimensional possibilities, the properties of carbon nanotubes are highly adjustable, or “tunable.” While many intriguing properties of carbon nanotubes have been identified, harnessing the properties of carbon nanotubes in practical applications requires scalable and controllable production methods that allow the features of the carbon nanotubes to be maintained or enhanced.

Due to their unique structure, carbon nanotubes possess mechanical, electrical, chemical, thermal and optical properties that make them well-suited for certain applications. In particular, carbon nanotubes exhibit superior thermal and electrical conductivity, high mechanical strength, good thermal stability and are also hydrophobic. In addition to these properties, carbon nanotubes may also exhibit useful optical properties. For example, carbon nanotubes may be used in light-emitting diodes (LEDs) and photo-detectors to emit or detect light at narrowly selected wavelengths. Carbon nanotubes may also prove useful for photon transport and/or phonon transport.

In accordance with various embodiments of the subject disclosure, nanofibers (including but not limited to carbon nanotubes) can be arranged in various configurations, including in a configuration referred to herein as a “forest.” As used herein, a “forest” of nanofibers or carbon nanotubes refers to an array of nanofibers having approximately equivalent dimensions that are arranged substantially parallel to one another on a substrate. FIG. 1 shows an example forest of nanofibers on a substrate. The substrate may be any shape but in some embodiments the substrate has a planar surface on which the forest is assembled. As can be seen in FIG. 1, the nanofibers in the forest may be approximately equal in height and/or diameter.

Nanofiber forests as disclosed herein may be relatively dense. Specifically, the disclosed nanofiber forests may have a density of at least 1 billion nanofibers/cm². In some specific embodiments, a nanofiber forest as described herein may have a density of between 10 billion/cm² and 30 billion/cm². In other examples, the nanofiber forest as described herein may have a density in the range of 90 billion nanofibers/cm². The forest may include areas of high density or low density and specific areas may be void of nanofibers. The nanofibers within a forest may also exhibit inter-fiber connectivity. For example, neighboring nanofibers within a nanofiber forest may be attracted to one another by van der Waals forces. Regardless, a density of nanofibers within a forest can be increased by applying techniques described herein.

Methods of fabricating a nanofiber forest are described in, for example, PCT No. WO2007/015710, which is incorporated herein by reference in its entirety.

Various methods can be used to produce nanofiber forests. For example, in some embodiments nanofibers may be grown in a high-temperature furnace, schematically illustrated in FIG. 2. In some embodiments, catalyst may be deposited on a substrate, placed in a reactor and then may be exposed to a fuel compound that is supplied to the reactor. Substrates can withstand temperatures of greater than 800° C. or even 1000° C. and may be inert materials. The substrate may comprise stainless steel or aluminum disposed on an underlying silicon (Si) wafer, although other ceramic substrates may be used in place of the Si wafer (e.g., alumina, zirconia, SiO₂, glass ceramics). In examples where the nanofibers of the forest are carbon nanotubes, carbon-based compounds, such as acetylene may be used as fuel compounds. After being introduced to the reactor, the fuel compound(s) may then begin to accumulate on the catalyst and may assemble by growing upward from the substrate to form a forest of nanofibers. The reactor also may include a gas inlet where fuel compound(s) and carrier gasses may be supplied to the reactor and a gas outlet where expended fuel compounds and carrier gases may be released from the reactor. Examples of carrier gases include hydrogen, argon, and helium. These gases, in particular hydrogen, may also be introduced to the reactor to facilitate growth of the nanofiber forest. Additionally, dopants to be incorporated in the nanofibers may be added to the gas stream.

In a process used to fabricate a multilayered nanofiber forest, one nanofiber forest is formed on a substrate followed by the growth of a second nanofiber forest in contact with the first nanofiber forest. Multi-layered nanofiber forests can be formed by numerous suitable methods, such as by forming a first nanofiber forest on the substrate, depositing catalyst on the first nanofiber forest and then introducing additional fuel compound to the reactor to encourage growth of a second nanofiber forest from the catalyst positioned on the first nanofiber forest. Depending on the growth methodology applied, the type of catalyst, and the location of the catalyst, the second nanofiber layer may either grow on top of the first nanofiber layer or, after refreshing the catalyst, for example with hydrogen gas, grow directly on the substrate thus growing under the first nanofiber layer. Regardless, the second nanofiber forest can be aligned approximately end-to-end with the nanofibers of the first nanofiber forest although there is a readily detectable interface between the first and second forest. Multi-layered nanofiber forests may include any number of forests. For example, a multi-layered forest may include two, three, four, five or more forests.

Nanofiber Sheets

In addition to arrangement in a forest configuration, nanofibers of the present application may also be arranged in a sheet configuration. As used herein, the term “nanofiber sheet,” “nanotube sheet,” or simply “sheet” refers to an arrangement of nanofibers where the nanofibers are aligned end to end in a plane. In some examples, this alignment can be accomplished via a drawing process (as described in PCT Publication No. WO 2007/015710, and incorporated by reference herein in its entirety) so that a longitudinal axis of a nanofiber within the sheet is parallel to a major surface of the sheet, rather than perpendicular to the major surface of the sheet (i.e., as in a “forest”). An illustration of an example nanofiber sheet is shown in FIG. 3 with labels of the dimensions. In some embodiments, the sheet has a length and/or width that is more than 100 times greater than the thickness of the sheet. In some embodiments, the length, width or both, are more than 10³, 10⁶ or 10⁹ times greater than the average thickness of the sheet. A nanofiber sheet can have a thickness of, for example, between approximately 5 nm and 30 μm and any length and width that are suitable for the intended application. In some embodiments, a nanofiber sheet may have a length of between 1 cm and 10 meters and a width between 1 cm and 1 meter. These lengths are provided merely for illustration. The length and width of a nanofiber sheet are constrained by the configuration of the manufacturing equipment and not by the physical or chemical properties of any of the nanotubes, forest, or nanofiber sheet. For example, continuous processes can produce sheets of any length. These sheets can be wound onto a roll as they are produced.

As can be seen in FIG. 3, the axis in which the nanofibers are aligned end-to end is referred to as the direction of nanofiber alignment. In some embodiments, the direction of nanofiber alignment may be continuous throughout an entire nanofiber sheet. Nanofibers are not necessarily perfectly parallel to each other and it is understood that the direction of nanofiber alignment is an average or general measure of the direction of alignment of the nanofibers.

Nanofiber sheets may be assembled using any type of suitable process capable of producing the sheet. In some example embodiments, nanofiber sheets may be drawn from a nanofiber forest. An image of an example of a nanofiber sheet being drawn from a nanofiber forest is shown in FIG. 4

As can be seen in FIG. 4, the nanofibers may be drawn laterally from the forest and then align end-to-end to form a nanofiber sheet. In embodiments where a nanofiber sheet is drawn from a nanofiber forest, the dimensions of the forest may be controlled to form a nanofiber sheet having particular dimensions. For example, the width of the nanofiber sheet may be approximately equal to the width of the nanofiber forest from which the sheet was drawn. Additionally, the length of the sheet can be controlled, for example, by concluding the draw process when the desired sheet length has been achieved.

Nanofiber sheets have many properties that can be exploited for various applications. For example, nanofiber sheets may have tunable opacity, high mechanical strength and flexibility, thermal and electrical conductivity, and may also exhibit hydrophobicity. Given the high degree of alignment of the nanofibers within a sheet, a nanofiber sheet may be extremely thin. In some examples, a nanofiber sheet is on the order of approximately 10 nm thick (as measured within normal measurement tolerances), rendering it nearly two-dimensional. In other examples, the thickness of a nanofiber sheet can be as high as 200 nm or 300 nm. As such, nanofiber sheets may add minimal additional thickness to a component.

As with nanofiber forests, the nanofibers in a nanofibers sheet may be functionalized by a treatment agent by adding chemical groups or elements to a surface of the nanofibers of the sheet and that provide a different chemical activity than the nanofibers alone. Functionalization of a nanofiber sheet can be performed on previously functionalized nanofibers or can be performed on previously unfunctionalized nanofibers. Functionalization can be performed using any of the techniques described herein including, but not limited to CVD, and various doping techniques.

Nanofiber sheets, as drawn from a nanofiber forest, may also have high purity, wherein more than 90%, more than 95% or more than 99% of the weight percent of the nanofiber sheet is attributable to nanofibers, in some instances. Similarly, the nanofiber sheet may comprise more than 90%, more than 95%, more than 99% or more than 99.9% by weight of carbon.

Nanofiber Yarns

Nanofiber sheets drawn from a nanofiber forest can be spun into nanofiber yarns. Nanofiber yarns can be single ply nanofiber yarns that are formed from the true-twisting or false-twisting of a single nanofiber sheet into a single strand nanofiber yarn. An example of this is shown in FIG. 5A and is described below in more detail. Multiple single ply nanofiber yarns can be plied together to form a multi-ply yarn. An example of this is shown in FIG. 5B and is described below in more detail.

In some examples, nanofiber yarns (and nanofiber sheets before being spun into a nanofiber yarn) can be optionally “densified” using a solvent. During densification, a solvent is applied to the nanofiber yarn (or nanofiber sheet). The solvent causes the individual nanofibers of the yarn (or nanofiber sheet) to draw closer to one another either upon application, upon removal of the solvent, or both, thus increasing the number of nanofibers per unit volume (i.e., density). This increase in density of the nanofiber yarn (or nanofiber sheet) can, for example, increase electrical conductivity, thermal conductivity, ultimate tensile strength, among other properties.

This process of applying, and subsequently removing, a solvent can also be used to coat at least one additional material onto surfaces of nanofibers within a nanofiber yarn (or sheet) and/or place the at least one additional material within the gaps between nanofibers. For example, nanoparticles or microparticles (e.g., silver, electrically conductive metals or semiconductors, magnetic metals or intermetallics) can be suspended in a solvent and carried into the gaps within a nanofiber yarn (or sheet) that are defined by the nanofibers of the yarn. The solvent can be removed, leaving the particles within the gaps formed by the nanofibers that form the nanofiber yarn and/or nanofiber sheet. For example, adding conductive particles can increase the electrical and/or thermal conductivity of the nanofiber yarn. Analogously, a polymer (or polymers, or a polymer mixed with nano or micro particles) can be infiltrated within the nanofiber yarn (or sheet). Upon removal of the solvent, the polymer (and any third material, such as nanoparticles) will remain within the nanofiber yarn (or sheet), and alter the properties accordingly. In some examples, the polymer can fill some of the gaps within the nanofiber sheet between, and defined by, nanofibers. In some examples, the polymer can form a conformal coating on surfaces of individual nanofibers within an interior of the nanofiber sheet. In still other examples, both of these configurations of polymer can be accomplished.

FIG. 5A illustrates the scanning electron microscope (SEM) image of a single ply, false twisted nanofiber yarn of the present disclosure. False twisting techniques are described in U.S. patent application Ser. No. 15/844,756, which is incorporated by reference herein in its entirety. In some examples, a single ply, false twisted nanofiber yarn such as the one illustrated in FIG. 5A, can have a cross-sectional diameter (taken perpendicular to a longitudinal axis of the yarn) within any of the following ranges: from 5 μm to 40 μm; from 5 μm to 30 μm; from 5 μm to 20 μm; from 10 μm to 30 μm; from 50 μm to 40 μm; from 25 μm to 35 μm, less than 40 μm, less than 30 μm, less than 20 μm, greater than 5 μm, greater than 10 μm or greater than 20 μm. In the specific example of the nanofiber yarn shown in FIG. 5A, the cross-sectional diameter is approximately 15 μm. The nanofiber yarn depicted in FIG. 5A has surface topography features that are less than 1 μm (and in some examples less than 0.1 μm) above or below an average location of the surface crossed a length and surface of the yarn.

FIG. 5B is an SEM micrograph of a multi-ply nanofiber yarn, in an example of the present disclosure. As indicated above, a multi-ply nanofiber yarn can be fabricated by plying together two or more single ply nanofiber yarns. These multi-ply yarns can have increased thermal, electrical, and mechanical properties in proportion to the increased number of fibers to transport heat and/or electricity, or bear a stress.

Thermochromic Nanofiber Yarn

As described herein, a nanofiber collection is a generic term that refers to nanofibers of any of a variety of configurations, including yarns, untwisted strands, sheets, forests, any of which can be densified or not densified. As indicated above, embodiments of the present disclosure can include a nanofiber collection to which two or more thermochromic materials have been applied. The combination of a nanofiber collection and the two or more thermochromic materials (which may include the polymer component that encapsulates or otherwise comprises a matrix for the thermochromic materials, as well as other materials such as solvents) is referred to herein as a thermochromic nanofiber collection. Many examples below are described in the context of a thermochromic nanofiber yarn, but this is for convenience of explanation. It will be appreciated that embodiments described below can be applied and/or adapted to other configurations of nanofiber collections.

Thermochromic nanofiber yarns of the present disclosure can display three or more different colors upon transitioning through thermochromic transition temperatures associated with each of the applied thermochromic materials. Generally, a single thermochromic material has one transition temperature and displays two colors. Below its transition temperature, the thermochromic material displays one color. Above its transition temperature, the thermochromic material changes to a different color. In examples described herein, when two thermochromic materials are mixed together, they can possess two transition temperatures and display three different colors. In other examples, more than two thermochromic materials can be selected to have different thermochromic transition temperatures mixed together to display more than three colors.

In some examples, the transition temperature can be a high temperature (e.g., greater than 40° C.) or in other examples, a low temperature (e.g., less than 5° C.). In some examples, transition temperatures range from −20° C. to +110° C. To accomplish a rapid response time of less than 100 milliseconds between color changes, embodiments of the present disclosure include carbon nanofiber yarns that are in direct contact with and/or proximate to a thermochromic layer. Because of the high thermal conductivity of nanofiber yarns, heat can be conducted to and/or conducted away from the thermochromic layer much more rapidly than in the absence of carbon nanofiber yarns.

In one or more embodiments described herein, the thermochromic material may be a pigment molecule, particle or ink that is applied directly to a nanofiber yarn or embedded within a solid or liquid matrix to produce a solid or liquid phase material that can be applied to a nanofiber yarn. Examples of suppliers of thermochromic materials include Chromadic Creations Inc. (SpectraBurst™) and the Atlanta Chemical Engineering Company LLC.

In some examples, embodiments of the present disclosure use relatively low electrical power input to transition the thermochromic materials to different colors. In one particular embodiment, a thermochromic transition can be caused in a 40-ply thermochromic yarn by applying 0.055 W/cm (+/−5%). The low power needed to cause one or more thermochromic transitions within embodiments of the present disclosure expands the possible technological applicability of thermochromic materials. That is, a 40 ply thermochromic yarn that can display three different colors upon transitioning between corresponding thermochromic transition temperatures can be powered by portable power sources such as AA, AAA, or even smaller (e.g., “button cell”) rechargeable or disposable batteries. In some other embodiments, the thermochromic materials can be mixed with one or more conductive materials to further lower the power usage by the yarns in transitioning to different colors.

In examples, a thermochromic nanofiber yarn can be a single ply yarn or a multi-ply yarn (e.g., the 40-ply yarn mentioned above). In some embodiments, thermochromic yarns can have a diameter greater than 5 μm.

In some embodiments, the thermochromic materials can be mixed with an actuating material. The thermochromic material and the actuating material in the mixture can be in the range of a ratio of 1:5 to 1:20. In a particular embodiment, the actuating material can be silicone rubber.

Method of Fabrication

FIGS. 6 and 7 illustrate an example system 600 and a corresponding example method 700 for fabricating a thermochromic nanofiber yarn. The individual structural components of the thermochromic nanofiber yarn fabricated according to the method 700 are explained below in more detail in the context of FIGS. 8, 9A, and 9B.

The system 600 includes a first thermochromic material applicator 604, a vacuum station 608, a yarn spinner 612, a first heating station 616, a second thermochromic material applicator 620, and a second heating station 624. In some other embodiments, the system 600 may not include a second thermochromic material applicator and/or a heating station.

The method 700 includes drawing 704 a nanofiber sheet 628 from a nanofiber forest (not shown). Drawing 704 of a nanofiber sheet from a nanofiber forest has been described above in the context of FIG. 4.

After drawing 704 a nanofiber sheet 628 from a nanofiber forest, a first thermochromic material 632 is applied 708 to the nanofiber sheet 628 to produce a nanofiber sheet 636 that includes the first thermochromic material 632. In one example, the first thermochromic material 632 can be applied by passing a nanofiber sheet 628 proximate to the first thermochromic material applicator 604, which may apply the first thermochromic material 632 from a reservoir 640 via a channel 644. The first thermochromic material 632 flows through the channel 644 onto the nanofiber sheet 628 as the nanofiber sheet 628 is drawn past a dispensing opening 648 defined at one end of the channel 644. In another embodiment, the nanofiber sheet 628 can be drawn through a container holding the first thermochromic material 632 in a liquid state. Various other techniques can be used to apply a thermochromic material to a nanofiber sheet, such as vapor deposition, spray coating, and dip coating. In either of the preceding examples, the first thermochromic material 632 can be applied as a suspension or solution that includes a thermochromic pigment or molecule, a solvent (or multiple solvents), and/or a polymer.

The rate and/or amount of first thermochromic material 632 applied can be selected in coordination with a speed at which the nanofiber sheet passes by the dispensing opening of the channel and a desired thickness (or viscosity) of the first thermochromic material 632 to be formed on the exposed surface of the nanofiber sheet.

Once dispensed, the first thermochromic material 632 may coat the top major surface of the sheet 628 and/or, in some examples, infiltrate into an inside portion of nanofiber sheet(s) that is between the major surfaces of the sheet(s).

Optionally, applying 708 the first thermochromic material 632 may include removing 710 some of the first thermochromic material 632 from an outside surface of the nanofiber sheet 628. In one embodiment, removal 710 can be performed by applying vacuum suction proximate to the nanofiber sheet 628 and its corresponding layer of first thermochromic material 632 via the vacuum station 608 while the first thermochromic material 632 is in a liquid or flowable state (that is, prior to solidification of the first thermochromic material 632). In one embodiment, the vacuum station 608 may include a collection chamber 652 and a vacuum channel 656 that defines an opening 660. In some examples, excess first thermochromic material 632 can be removed from an outside surface of the nanofiber sheet 628 while leaving at least a portion of first thermochromic material 632 disposed within gaps between nanofibers in an inside portion (e.g., between the major surfaces) of the nanofiber sheet 628.

It will be appreciated that other mechanisms can also be used to remove 710 excess first thermochromic material 632 from the nanofiber surface. For example, an impingement mechanism can be used to scrape off the surface coating (e.g., a squeegee or doctor blade). In another example, a stream or focused bursts of fluid (e.g., compressed air, solvent) can be used to remove the excess first thermochromic material 632. In another example, excess first thermochromic material can be evaporated or otherwise removed 710 by heating. In yet another example, excess first thermochromic material 632 is removed by laser ablation. Other types of removal methods will be appreciated in light of this disclosure.

After applying 708 the first thermochromic material 632 onto the nanofiber sheet 628 and optionally removing 710 extra material from the outside surface of the nanofiber sheet 636, the nanofiber sheet treated with the first thermochromic material 632 can be twisted 712 into a first thermochromic nanofiber yarn 664 by passing the nanofiber sheet 636 through a yarn spinner 612. Twisting a nanofiber sheet into a nanofiber yarn has been described above in the context of FIGS. 5A and 5B.

Optionally, the first thermochromic nanofiber yarn 664 can be heated 716 by a first heating station 616 to cure the first thermochromic material 632 (or accelerate evaporation of a solvent) prior to applying 720 a second thermochromic material 668. The temperatures of the first thermochromic nanofiber yarn 664 achieved at the heating station 616 and the duration of the heating 716 are determined at least in part by the material chosen for a first thermochromic material. For example, a temperature can be selected below a degradation temperature of the first thermochromic material, below a flash point of solvent used to apply the first thermochromic material, or below a glass transition temperature or charring temperature of a polymer matrix in which the first thermochromic material is applied. In some embodiments, the first thermochromic nanofiber yarn 664 can be heated 716 from 50° C. to 110° C. to encourage evaporation of the solvent(s) or to encourage flow of a polymer matrix over and into the nanofiber sheet 636. For example, in some embodiments, heating 716 can cause the first thermochromic material 632 to flow into inside portion of the first thermochromic yarn 664 thereby exposing the inner surface of the yarn with thermochromic materials. In some embodiments, heating 716 can be applied by radiant heat (e.g., a heating element in the first heating station 616), inductive heating through applied electromagnetic fields (emitted by the first heating station 616) or by joule heating by applying current directly to the nanofiber yarn.

After heating 716 the first thermochromic nanofiber yarn 664 (simply, yarn 664), a second thermochromic material 668 can be applied 720 to produce a second thermochromic nanofiber yarn 672. Generally, a second thermochromic material 668 is compositionally different from a first thermochromic material. That is, the first thermochromic material 632 is formulated from different chemical groups, different elements, and/or different structural arrangements of chemical groups and elements than those used to formulate the second thermochromic material 668.

Application 720 of the second thermochromic material 668 can occur by passing the first thermochromic nanofiber yarn 664 proximate to a second material applicator 620 where second thermochromic material 668 is applied. In one embodiment of method 700, the second material applicator 620 includes a reservoir 676 and a channel 680. The second thermochromic material 668 flows through channel 680 onto the first thermochromic yarn 664 as the yarn 664 is drawn past a second dispensing opening 684 defined by one end of the channel 680. In one embodiment, the nanofiber yarn 664 is pulled through the second thermochromic material 668 in a liquid state as it is dispensed from the second material applicator 620. Other application methods for application of second thermochromic material 668 may also be used as indicated above. These other application methods include, but are not limited to, vapor deposition, spray coating, and dip coating.

The second thermochromic material 668 can be optionally heated 724 or otherwise solidified (e.g., by removing a solvent) by a second heating station 624. In some embodiments, the second thermochromic nanofiber yarn 672 can be heated 724 from 50° C. to 110° C. for evaporating the solvent(s), as described above in the context of optional heating 716.

Additional techniques for applying multiple materials on a nanofiber collection (which includes nanofiber sheets and yarns) are described in U.S. patent application Ser. No. 16/103,102, which is incorporated herein by reference in its entirety.

Alternatively, in some example embodiments, a first thermochromic material 632 and a second thermochromic material 668 can be mixed together in equal parts to form a single homogeneous layer of thermochromic materials, in contrast to the layer-by-layer application used in method 700.

In some embodiments, the yarn infiltrated with the first 632 and the second 668 thermochromic materials can be further plied 728 with an additional thermochromic yarn which includes at least one thermochromic material.

FIG. 8 is a schematic illustration of a thermochromic nanofiber yarn 800 capable of displaying three different colors within three different temperature ranges, in an example of the present disclosure. The thermochromic nanofiber yarn 800 includes a nanofiber collection (e.g., nanofiber yarn 628), a first thermochromic material 632 and a second thermochromic material 668. In some embodiments, the first thermochromic material 632 and the second thermochromic material 668 can be applied to the nanofiber collection in two separate layers as shown and described in the context of FIGS. 6 and 7, and further described below in the context of FIG. 9A. In some other embodiments, the first 632 and the second thermochromic materials 668 can be mixed together in any proportion (e.g., equal parts or some other proportion) and the resulting homogeneous mixture can be applied to the nanofiber collection in a single layer as shown and described below in the context of FIG. 9B.

The thermochromic yarn 800 can display three distinct colors at three corresponding different temperature ranges. The temperature ranges include a first temperature range T1, a second temperature range T2, and a third temperature range T3. FIG. 8 illustrates these three distinct colors as shown on the nanofiber yarn 800, which for the purposes of illustration is presumed to have three different regions, each of which is maintained at one of the three different temperatures T1, T2, and T3. Brackets in FIG. 8 indicate the areas of the nanofiber yarn 800 that correspond to these temperatures. The colors corresponding to each of these three temperatures T1, T2, and T3 are indicated by three different patterns (including the unshaded region corresponding to temperature T1).

In the examples shown in FIG. 8, the first temperature range T1 is below the transition temperatures of both the first and the second thermochromic materials, the second temperature range T2 is above the transition temperature of the first thermochromic material but below the transition temperature of the second thermochromic material, and the third temperature range T3 is above the transition temperatures of both of the thermochromic materials.

In a specific example of the thermochromic yarn 800, the first temperature range T1 is 0° C.-30° C., the second temperature range T2 is 31° C.-44° C., and the third temperature range T3 is above 45° C. At 0° C.-30° C., both the first and the second thermochromic materials can maintain colors which are blue and red, respectively, because in this example, 30° C. is below the transition temperatures of both of the example thermochromic materials. As a result, from 0° to 30° C., the first color shown by the thermochromic yarn 800 in region T1 is magenta because of the combination of blue and red. In the region of the yarn 800 heated to T2 (e.g., from 31° C. to 44° C.) the first thermochromic material transitions to a white color (having been heated beyond the corresponding thermochromic transition temperature of 31° C.). The second thermochromic material retains its first thermochromic color (red) because the T2 temperature range in this example (31° C. to 44° C.) is below the transition temperature of the second thermochromic material. As a result, the color of the thermochromic yarn 800 in the region T2 appears pink from the combination of white (of the first thermochromic material above its transition temperature and red (of the second thermochromic material below its transition temperature). Upon heating the thermochromic yarn 800 above 45° C. in the region T3, the second thermochromic material exceeds its transition temperature and thus transitions from red to white. As a result, at 45° C. and above, the T3 region of the thermochromic yarn 800 in FIG. 8 appears white.

FIG. 9A is a cross-sectional view of a thermochromic nanofiber yarn in which two separate layers of thermochromic materials have been applied to a nanofiber collection, in an example. In an embodiment, a thermochromic nanofiber yarn 900 includes an outer layer of nanofiber yarns 904 at an exterior surface of the yarn 900, a first thermochromic material 916, and a second thermochromic material 920. In some embodiments, the outer layer of nanofiber yarns 904 can be just the outer surface of a plurality of nanofibers 908. In some other embodiments, the outer layer of nanofiber yarns 904 can have an additional material such a polymer or another material applied to the outer surface of the plurality of nanofibers 908. In one embodiment, the outer layer of nanofiber yarns 904 includes the plurality of nanofibers 908 and a plurality of inter-fiber spacings 912 defined by the plurality of nanofibers 908.

FIG. 9B is a cross-sectional view of the thermochromic nanofiber yarn of FIG. 8 in which a single layer comprising both of the two thermochromic materials have been applied to a nanofiber collection, in another example of the present disclosure. In this alternate embodiment, a thermochromic nanofiber yarn 950 includes an outer layer of nanofiber yarns 904 and a single, compositionally uniform layer of thermochromic materials 954. In one embodiment, the layer of thermochromic materials 954 can be a homogeneous mixture of two equal parts of the first thermochromic material and the second thermochromic material.

It will be appreciated that the approaches described above can be adapted to produce any number of colors by mixing two or more thermochromic materials according to the color desired.

Further Considerations

The foregoing description of the embodiments of the disclosure has been presented for the purpose of illustration; it is not intended to be exhaustive or to limit the claims to the precise forms disclosed. Persons skilled in the relevant art can appreciate that many modifications and variations are possible in light of the above disclosure.

The language used in the specification has been principally selected for readability and instructional purposes, and it may not have been selected to delineate or circumscribe the inventive subject matter. It is therefore intended that the scope of the disclosure be limited not by this detailed description, but rather by any claims that issue on an application based hereon. Accordingly, the disclosure of the embodiments is intended to be illustrative, but not limiting, of the scope of the invention, which is set forth in the following claims. 

What is claimed is:
 1. A method comprising: drawing a nanofiber sheet from a nanofiber forest; applying a first thermochromic material to the nanofiber sheet; twisting the nanofiber sheet with the first thermochromic material into a first thermochromic nanofiber yarn; and applying a second thermochromic material to the first thermochromic nanofiber yarn to produce a second thermochromic nanofiber yarn.
 2. The method of claim 1, further comprising heating the first thermochromic nanofiber yarn before applying the second thermochromic material.
 3. The method of claim 2, wherein heating the first thermochromic yarn comprises heating to a first temperature of from 90° C. to 115° C.
 4. The method of claim 1, further comprising heating the second thermochromic nanofiber yarn after applying the first thermochromic material and the second thermochromic material.
 5. The method of claim 4, wherein heating the second thermochromic yarn comprises heating to a second temperature of from 90° C. to 115° C.
 6. The method of claim 1, wherein the first thermochromic material is applied to the nanofiber sheet as a solution of a first solvent and the first thermochromic material.
 7. The method of claim 1, wherein the second thermochromic material is applied to the first thermochromic nanofiber yarn as a solution of a second solvent and the second thermochromic material.
 8. The method of claim 1, wherein the first thermochromic material has a first transition temperature.
 9. The method of claim 8, wherein the second thermochromic material has a second transition temperature different from the first transition temperature.
 10. The method of claim 9, wherein the second thermochromic nanofiber yarn displays: a first color below the first transition temperature; a second color different from the first color between the first transition temperature and the second transition temperature; and a third color different from the first and second colors above the second transition temperature.
 11. The method of claim 1, wherein a diameter of the first thermochromic nanofiber yarn and the second thermochromic nanofiber yarn is at least 5 microns.
 12. The method of claim 1, further comprising: drawing an additional nanofiber sheet from the nanofiber forest; applying a third thermochromic material to the additional nanofiber sheet; twisting the additional nanofiber sheet with the third thermochromic material into a third thermochromic nanofiber yarn; and plying the third thermochromic nanofiber yarn with the second thermochromic nanofiber yarn to produce a multi-ply thermochromic nanofiber yarn.
 13. The method of claim 12, wherein the third thermochromic material has a third transition temperature different from a first transition temperature and a second transition temperature.
 14. The method of claim 1, further comprising adding a conductive material to the nanofiber sheet.
 15. The method of claim 1, further comprising adding a metallic nanoparticle to the nanofiber sheet.
 16. A method comprising: drawing a nanofiber sheet from a nanofiber forest; applying a mixture of a first thermochromic material and a second thermochromic material to the nanofiber sheet; and twisting the nanofiber sheet with the first and the second thermochromic materials into a thermochromic nanofiber yarn.
 17. The method of claim 16, wherein the mixture of the first thermochromic material and the second thermochromic material is homogeneous.
 18. The method of claim 17, the mixture comprises equal parts of the first and the second thermochromic materials.
 19. The method of claim 18, wherein the mixture of the first thermochromic material and the second thermochromic material is applied to the nanofiber sheet in a single layer. 